The long-term objective of this study is to isolate temperature- sensitive mutants of a defective murine RNA tumor virus, the Moloney sarcoma virus, as a means of studying the viral gene(s) necessary for cell transformation and virus replication. The specific aims are: (1) Isolation of temperature-sensitive mutants. (2) Using these mutants the cistrons in the viral genome which are functional in cell transformation and virus replication will be determined. (3) To determine whether or not this virus is capable of genetic recombination using the temperature-sensitive mutants. (4) To examine specific biochemical parameters of mutant viruses which may be necessary for cell transformation and virus replication to occur.